1. Field of the Invention
The present invention is related to the field of digital image processing and more particularly to a method and associated apparatus for improving the tone and color reproduction of print and slide images which are photographed onto color negative film, scanned by a negative scanner and transferred onto Photo Compact Disc (hereinafter referred to as Photo CD).
2. Description of Related Art
Eastman Kodak Co., the assignee of the present invention, has been developing a "Photo CD" system that is illustrated in block form in FIG. 1. The system uses a film scanner 12 to scan 35 mm photographic film strips 10. The film scanner 12 converts sensed light, generally analog in nature, into digital signals and forwards these signals to a computer workstation 14. These digital image signals are processed and stored or passed directly to a CD recorder (writer) 16 by the operation of the workstation 14. The writer causes the digitized image data to be recorded on a write-once compact disc. A CD player 20 is used to read the disc and to generate electrical signals for driving a TV display 22 or a hard copy printer, such as a thermal printer 24.
Some potential consumer and industrial Photo CD users will be interested in scanning photographic prints, hardcopy documents, or slides, rather than 35 mm film. The Photo CD system is designed so that the outputs from color negative film scanners, color transparency (slide) film scanners, and reflection (print) scanners are all converted to a common image data metric. The conversion technique is described in the cross-referenced patent application entitled "Methods and Associated Apparatus for Forming Image Data Metrics Which Achieve Media Compatibility for Subsequent Imaging Applications" by Giorgianni et al.
FIGS. 2A and 2B illustrate conversions from various inputs to a common data metric. In the conversion shown the input signals are all R,G,B image bearing signals produced, for example, by scanning an image from a negative or transparency film 30A and 30B, respectively, with a transmission scanner 32, and transforming by transforms 34 the formed R,G,B signals to an image manipulation or storage metric 36 approximating the relative trichromatic exposure values that each input film received when it captured the original image. In a like manner reference medium descriptors and reference output device code values may also be used.
In FIG. 3 color patches (exposure patterns), chosen to adequately sample and cover the useful exposure range of the film being calibrated, are created by an exposing character generator 40 generating signals that are fed to an exposing apparatus 42. The exposing apparatus 42 produces known trichromatic exposures on the film 30C to create test images consisting of approximately 400 color patches. The test images may be created by using a variety of methods appropriate for the application. These methods include, but are not limited to, exposing using an exposing apparatus such as a sensitometer, using the output device of a color imaging apparatus, or using test objects of known reflectances illuminated by known light sources and calculating trichromatic exposure values. If input films of different speeds are used, the overall red, green and blue exposures must be properly adjusted for each film in order to compensate for the relative speed differences between the films. Thus, each film receives equivalent exposures, appropriate for its red, green, and blue speeds. Exposed film is processed chemically and the film patches are read by the transmission scanner 32 which produces the R,G,B image bearing signals corresponding to each color patch. A transform apparatus 44 creates a transform 34 relating the R,G,B image bearing signal values for the film's test colors to the known R',G',B' exposures of the corresponding test colors.
FIGS. 4A, 4B and 4C are simplified flowcharts of the methodologies described in the Giorgianni et al patent, showing how the original scene is photographed onto negative film or slide film, and how the film, or a print made from the film, is scanned. Note that each of the three flowcharts includes only one film step.
Some Photo CD customers having prints or slides to scan may find it impossible or inconvenient to directly scan their original images. For example, print or slide scanners may not be as readily available as negative scanners. They may therefore chose to rephotograph their prints or slides onto color negative film, and have the developed "duplicate" or "cascaded" film images scanned. The inventors of the present invention have recognized that if the Photo CD scanner is calibrated using only the methods described in the Giorgianni et al application,the result will be images with lower color saturation and higher contrast than desired. This occurs because the prior art calibration techniques are not designed to correct for the "cascading" of two photographic stages, but instead correct only for the second stage. Since the first photographic stage has a non-unity gamma as described in an article by Hunt, R. W. G. et. al, entitled "The Tone Reproduction of Colour Photographic Materials", Journal of Photographic Science, Vol 17, 1969, pp. 198-204, and a different (usually reduced) relative metric chroma compared to the scene, the tone and color reproduction from these applications employing two "cascaded" film steps will be very different from the tone and color reproduction of the applications utilizing only a single film step. What is needed is a method for improved correction of the tone and color reproduction of "cascaded" negative film images photographed from prints or slides, in order to provide improved picture quality.